Atherosclerosis develops from a complex interaction of lipoproteins, plasma factors (platelets, growth factors, etc.) and cells of the arterial wall. Although the cellular and molecular events responsible for the development of atherosclerosis are poorly understood, atheroslcerotic lesions are characterized by the presence of lipid-laden foam cells which appear to be derived from macrophages. Macrophages are immunocompetent cells whose biochemical functions are altered by their state of activation. It is not known what relationship lipid accumulation in macrophages has to the development of atherosclerosis, i.e., is it contributory or protective, nor is it known whether macrophages present in atherosclerotic lesions are resident (unstimulated) or activated cells. Given the present understanding of the cellular mechanisms of atherosclerosis and of macrophage functions, there is a variety of interactions of lipoproteins with macrophages which may be modified by macrophage activation and, conversely,a variety of macrophage functions which may be modified by lipoprotein uptake. The major goal of this proposal is to explore systematically the effects of macrophage activation on the expression of lipoprotein receptors, lipid accumulation and cellular enzyme activities and to use this information to define the state of activation of macrophages present in atherosclerotic lesions. To perform these experiments, macrophages will be cultured from peritoneal exudates of mice which have been inoculated with various agents in order to obtain cells whose states of activation span the spectrum from unstimulated to fully activated. In some experiments nonactivated macrophages will be treated in vitro with agents which will convert them to an activated state. Once macrophages of varying activation are obtained, the expression lipoprotein receptors will be assessed by measuring the binding and degradation of 125I-VLDL, 125I-Beta VLDL, 125I-LDL and 125I-chemically modified LDL. In parallel, the changes in cellular lipid metabolism during activation will be assessed by examining the activities of HMG CoA reductase, ACAT, neutral cholesteryl esterase, neutral triglyceride lipase and LPL. Moreover, the ability of lipoproteins to regulate these enzyme activities, as well as the secretory activities of macrophages, will be determined. Furthermore, the mechanisms involved in the macrophage's regulation of cellular lipid accumulation under different states of activation will be determined. Additionally, atherosclerosis will be induced in mice and rats by cholesterol feeding and the state of activation of the macrophages in their lesions assessed. The results of this proposal should increase the fundamental understanding of the interaction between lipoproteins and macrophages.